Generation of sinusoidal waveforms is a common function of circuitry used in telecommunications. For example, dual-tone multi-frequency (DTMF) dialing signals are composed of pure sinusoidal waveforms of two different frequencies. Other uses of sinusoidal waveforms are found in such circuits; commonly required are waveforms whose amplitude decays with time.
Various techniques are employed to generate sinusoidal waveforms: dedicated digital/analog circuitry such as is employed in American Microsystems' model S25089 tone generator; memory table look-up of sine values; and triangular and trapezoidal approximations. These three methods are described in U.S. Pat. No. 4,563,548 entitled "Method and Circuit for Telephone Dialing Signal Generation" and such description is incorporation herein by reference.
The digital/analog circuitry makes use of dedicated circuits to generate sinusoidal waveforms. These dedicated circuits take up valuable die area on the integrated circuit chip. Furthermore, they are generally fixed in terms of the functions they perform because of their use of analog circuit components.
The latter two techniques are more desirable because they are typically purely digital in nature and accordingly can be programmed to perform a variety of functions. However, the memory look-up still requires dedicated read-only memory (ROM) to store sine values as well as amplitude templates for decaying waveforms. Furthermore, it is still somewhat inflexible because of the use of only selected values of amplitude information. The triangular and trapezoidal technique is considerably more flexible and efficient of die area as it can be programmed to run on an arithmetic processor usually incorporated on telecommunications chips. However, approximation of sinusoidal waveforms by straight-line segments produces undesirable harmonics in the resulting waveforms and only the fundamental frequency is correct. Additional filtering is therefore necessitated by this technique.
What is needed is a method of generating pure and decaying sinusoidal waveforms which makes full use of the arithmetic processor commonly incorporated in telecommunications integrated circuits. Thereby not requiring circuitry dedicated solely to generation of sinusoidal waveforms and producing such waveforms accurately.